You could do worse than to start using --only-binary=:all: by default. (It's even been proposed as default behaviour: https://github.com/pypa/pip/issues/9140) Even if you can't actually install that way, it will point out the places where sdists are needed.
In principle, separate metadata availability should still at least be possible for most sdists eventually. But I'm not the one calling the shots here.
Should I expect that to download only metadata and not whole wheels/sdists for everything? Or does that depend on everything in my requirements file being available as a wheel?
In principle, separate metadata availability should still at least be possible for most sdists eventually. But I'm not the one calling the shots here.